Ground-based navigation signals for aircraft navigation have been used since the mid 1940's. Broadly, these navigation signals are carried through radio-frequency (RF) transmissions, e.g., very-high frequency (VHF) transmissions, emitted by ground stations for purposes of guiding aircraft along prescribed routes or in particular directions. Examples of short-range, local-positioning navigation systems that use such signals include VHF omni-directional radio range (VOR) systems, instrument landing system localizer (Localizer) systems, and instrument landing system glideslope (Glideslope) systems. Nearly all older-model aircraft were configured to receive navigation signals emitted from ground stations of such local-positioning navigation systems. The navigation signals could be interpreted by an aircraft's onboard instruments to provide an indication to a pilot of the aircraft as to whether the aircraft was travelling on a proper bearing or flight track. For example, VOR systems would permit a pilot to stay on a prescribed course based on navigation signals received from a network of fixed ground station. On the other hand, Localizer and/or Glideslope systems were generally used to assist a pilot in landing his/her aircraft on a runway based on navigation signals received from a fixed ground station positioned on or near the runway.
Generally, to make use of such local-positioning navigation systems, aircraft included receiver units that could receive the RF transmissions emitted by the ground stations. The receiver units were often large components installed within the aircraft, generally positioned behind the instrument panel and/or in the nose of the aircraft. The receiver units were generally configured to remove carrier signals from the received RF transmissions so that the resulting navigation signals could be sent to a course direction indicator (CDI) installed within the instrument panel of the aircraft for visual observation by the pilot. Most older (and some newer) CDIs included an integral converter unit that would decode the navigation signals obtained from the receiver unit and, based on the decoded navigation signal, drive a needle on a display unit of the CDI. CDIs with integral converter units are referred to herein as composite CDIs. Integral converter units for composite CDIs require that the navigation signals received from the receiver units be in the form of a composite navigation signal, which is a navigation signal composed of two or more individual signals.
A CDI, such as one of those described above, would generally be integrated within an aircraft's instrument panel, such that the display unit (including the needle) of the CDI would be visible to the pilot of the aircraft. Some aircraft would include multiple CDIs for each of VOR and Localizer/Glideslope systems. Alternatively, some aircraft would include a single CDI configured to function as an indicator for both VOR systems and Localizer/Glideslope systems. The position of the needle of a CDI incorporated within an aircraft can be used to provide an indication as to whether the aircraft is in a proper position and/or is on a proper course or bearing with respect to an intended waypoint. As noted above, the position of the needle could be driven by the navigation signals embedded within the RF transmission received by the receiver unit. Specifically, the RF transmissions would generally be received by the receiver unit, which would remove the carrier signal and send the resulting composite navigation signal to the converter unit of the CDI. The converter unit could decode the navigation signal and generate a control signal, which would drive the needle of the CDI.
Given the above, a CDI would typically allow a pilot of an aircraft to select a desired course, and thereafter continuously obtain relative bearing or flight track deviation information from the CDI by observing the position of the needle of the CDI. For example, after the pilot sets a desired course, if the needle of the CDI indicates that the aircraft is positioned to the left of the intended course, the pilot can maneuver the plane towards the right until the needle is centered, which indicates that the aircraft is back on its intended course.
Area navigation (RNAV) systems, such as Global-Positioning System (GPS) systems or Long-Range Navigation (LORAN) systems, have recently begun replacing local-positioning navigations systems (e.g., VOR, Localizer, Glideslope) for aircraft navigation. GPS navigation systems began to increase in popularity in the 1990's. In the early 2000's, GPS navigation systems were approved as sole means of aircraft navigation in conjunction with Wide Area Augmentation System (WAAS).
To reduce the complexity and cost of previously-used composite CDIs, which required use of receiver units and integral converter units, RNAV systems (e.g., GPS and LORAN) allowed for the use of simplified CDIs that lacked integral converter units. These simplified CDIs would generally include just a display unit that could be configured to receive simplified information (e.g., a +/−150 my DC control signal) from a source so as to drive the needle of the simplified CDI. For instance, these simplified CDIs could be connected directly to GPS units so as to display GPS data. Alternatively, some newer local-positioning navigations system receiver units, such as for VOR, Localizer, and/or Glideslope, can incorporate integral converter units directly within the receiver units. As such, if connected to a local-positioning navigations system receiver unit with integral converter unit, a simplified CDIs can receive a control signal so as to display local-positioning navigation system data.
However, older CDIs that include integral converter units (i.e., composite CDIs) have become less prevalent in the aviation industry because most radios now include the composite converter circuitry within the RF receiver. Such composite CDIs have been left to function only in conjunction with older local-positioning navigation system receiver units, or they have been removed altogether from aircraft and deemed near valueless.